Here is one sample scenario for
ORA-19811 error. I connected to target as / and tried to execute the change
command. It is giving the ORA-19811 error. You should connect to recovery
catalog database to execute the change command.

Also, you cannot use the KEEP for the
backupset in Flash Back Recovery area (FRA). You need to copy the backupset
outside of the FRA location and use the catalog backuppiece command whenever
you wanted to restore it.

Shared database link help you to limit the network connections between the local and remote servers. For a shared database link need 10 shared server processes in local database and also it requires upto 10 network connections to the remote server.